Prior to this change there were only 3 instances of `STD(IN|OUT|ERR)`
versus 74 instances of `$std(in|out|err)` in the Homebrew code base. The
latter variant is also strongly suggested by bbatsov's Ruby Style Guide.
This collects all violations for each formula in a single place, instead
of doing `brew style` outputs for all formulae first, and then the other
audit checks.
Closes#112.
Signed-off-by: Andrew Janke <andrew@apjanke.net>